U.S. Pat. No. 4,378,159 to Galbraith relates to a scanning laser contaminant and defect detector which uses a reflective spherical shell sector to collect light from a reflective surface under test. A photoelectric detector in the shell facing the test surface detects changes in scattering, indicative of particles which have different scattering properties than the test surface.
U.S. Pat. No. 4,243,891 to Dobler et al. teaches optical hole detection by means of light. A prism is used to uniformly illuminate a web, such as photographic film, with an optical detector on the underside of the web to detect holes in the web.
U.S. Pat. No. 4,380,032 to Pfost teaches the scanning of magnetic tape as the tape moves past an optical station looking for either reflective color marks on the tape or light transmissive holes in the tape.
One of the problems encountered in quality control of thin films, plates, wafers, foil, and other optically reflective and transmissive members is the accumulation of surface particles, as well as the occurrence of microscopic pinholes which are not intended. Such pinholes may arise from manufacturing problems or from handling. In very thin test surfaces, or surfaces having a low-to-medium optical density, an inspection beam will penetrate the test surface whether a hole is present or not. Therefore, conventional detectors will not yield meaningful data with respect to holes in the material because of the continuous presence of a detected beam through the test surface. Scientific and industrial users of these thin products often impose quality requirements which must be checked prior to using them in experiments or in manufacturing processes. For example, in semiconductor manufacturing, a substantial amount of value is added to a thin silicon wafer since the wafer is the substrate on which integrated circuits are built. Most semiconductor manufacturers have a need to know whether a wafer meets a quality specification for the starting material. Presently, a wafer or similar material can be checked for particles, but there is no single apparatus which also tests for holes, especially where the wafer is transmissive to an inspection beam.
An object of the present invention is to devise an optical instrument for simultaneously monitoring particles an optically transmissive member and checking the surface for holes.